1. Technical Field
The present invention relates to wireless communications and, more particularly, radio front end circuitry for up-converting and down-converting transmission signals.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier stage. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier stage amplifies the RF signals prior to transmission via an antenna.
Typically, the data modulation stage is implemented on a baseband processor chip, while the intermediate frequency (IF) stages and power amplifier stage are implemented on a separate radio processor chip. Historically, radio integrated circuits have been designed using bi-polar circuitry, allowing for large signal swings and linear transmitter component behavior. Therefore, many legacy baseband processors employ analog interfaces that communicate analog signals to and from the radio processor.
The demand for high performance universal frequency synthesizers is growing with the increasing performance and integration requirements of wireless communications systems such as cellular telephones and personal digital assistants (PDAs). The driving forces are lower cost, smaller form factors, and lower power consumption for consumer end products. Fractional-N phase locked loop (PLL) frequency synthesis is a popular indirect frequency synthesis method for high performance applications such as cellular telephony due to its agility and the ability of synthesizing frequencies over wide bandwidths with narrow channel spacing. For example, in GSM cellular telephony, one pair of RF bands, i.e., transmit (TX) and receive (RX) bands, consists of the frequencies 880.2 MHz-914.8 MHz and 925.3 Mhz-959.9 MHz, respectively. The four RF bands of GSM are also referred to as the GSM850, GSM900, DCS, and PCS bands, respectively. Typical architectures for such radios include a single oscillator that is divided to produce the specified output frequency that is required for an outgoing radio frequency transmission. The dividend, however, often is not a whole number and thus the term “Fractional-N” refers to a non-integer dividend that is used in a PLL to produce the desired output frequency. While Fractional-N PLLs are known, there is an ever increasing need for Fractional-N PLLs that are more efficient or are optimized for a particular application.